A hot melt adhesive is a solvent-free adhesive and has features enabling of instant bonding and high-speed bonding since the adhesive exhibits adhesion after the adhesive is melted by heating, applied to an adherent and then solidified by cooling. For example, the hot melt adhesive is now widely used in paper processing, woodworking, sanitary materials, electronic fields, and automotive interior materials.
Patent Document 1 discloses a hot melt adhesive for automotive interior materials, containing an α-olefin-based polymer as a main component (see, for example, component (A) of [Claim 1]). A door trim and a roof trim, which are automotive interior materials, are frequently obtained by forming a polyolefin such as a propylene resin. Therefore, the hot melt adhesive disclosed in the same document contains a large amount of the α-olefin-based polymer, thus improving adhesion to the automotive interior material.
Furthermore, a crystalline polypropylene having a melting point of 120° C. or higher is blended in the hot melt adhesive disclosed in the same document (see component (B) of [Claim 1]). The crystalline polypropylene resin enables the hot melt adhesive to become more suitable for applications such as automotive interior materials, as a result of an improvement in heat resistance ([0024]).
However, polyester is sometimes used in the automotive interior material such as a skin of the ceiling section. The hot melt adhesive of Patent Document 1 is excellent in adhesion to a polyolefin substrate, but exhibits insufficient adhesion to the polyester substrate. The hot melt adhesive of Patent Document 1 is also excellent in heat resistance; on the other hand, it is likely to undergo embitterment, leading to insufficient peel strength.
In the automotive field, due to growing concern about environmental problems, the replacement of conventional raw materials derived from petroleum by natural materials, vegetable materials, and biodegradable materials has made progress. From the viewpoint of environmental problems such as global warming due to an increase in emission of a carbon dioxide gas, a study has also been made on a polylactic acid-based resin using no petroleum as a raw material, relating to automotive interior materials and adhesives.
Patent Document 2 discloses a hot melt adhesive containing a polylactic acid-based resin and polybutylene succinate or polyethylene succinate. Patent Document 3 discloses a hot melt adhesive containing polylactic acid and a polyvinyl alcohol-based resin. Patent Document 4 makes mention of a hot melt adhesive composition which is a composition containing, as main components, a thermoplastic resin and a tackifier, one or both of which contain(s) a polylactic acid, or a lactic acid copolymer resin derived from lactic acid and other hydroxycarboxylic acids.
A polylactic acid-based hot melt adhesive tends to be inferior in tackiness, adhesion, thermal stability, heat resistance, and the like as compared with a polyolefin-based hot melt adhesive. The polylactic acid-based hot melt adhesives disclosed in Patent Documents 2 to 4 were not suited for application for automotive interior materials because of particularly poor adhesion to a polyolefin substrate used in automotive interior materials, and the like.
A method for blending a polyolefin in the polylactic acid-based hot melt adhesive is proposed as means for enhancing adhesion to the polyolefin substrate. However, polylactic acid had a problem that it is hard to be mixed with the polyolefin, is also inferior in compatibility with the “tackifier resin” which is an additive of the hot melt adhesive, and has poor thermal stability.